GB2098733A - Automatic noise measuring equipment - Google Patents

Abstract

An automatic noise dosimeter is described which totals exposure to varying noise levels and can initiate an alarm e.g. if the maximum daily permitted exposure is reached. A microphone (1) at the point where it is desired to monitor exposure has its output when greater than a predetermined threshold fed to a number of comparator circuits (Comp 1-7) which correspond to regions of increasing noise level. Using suitable circuitry (18), the total exposure at each level is calculated and summed on a continuous real time basis and the net noise dosage can be continuously displayed (at 14). <IMAGE>

Description

SPECIFICATION Automatic noise measuring equipment This invention relates to automatic noise measuring equipment.

In recent years it has been appreciated that exposure to noise can have damaging consequences on personnel who have to operate in noisy environments e.g. near machinery. Furthermore it has become apparent that the amount of damage is related to the cumulative amount of noise to which personnel are exposed. These discoveries have lead in turn to the establishment of noise standards which are applicable throughout industry and which set out precise limits of the amount of time to which a person exposed to noise may be exposed to various levels of noise. For example, O.S.H.A.Paper Sl-74 sets out permissible daily noise exposures for personnel as follows: Sound Level in dB(A) Maximum daily exposure (hours) 85 16 90 8 95 4 100 2 105 1 110 0.5 115 0.25 In order to ensure that regulations protecting workers are being complied with, relevant authorities have, in the past, had to resort to costly and time consuming recording of the varying noise level through a working day and subsequent complex analysis of the record so made. We have now designed apparatus for measuring noise exposure directly.

According to the present invention there is provided apparatus for detecting when noise exposure at a location, over a given period of time, exceeds a predetermined level, which apparatus comprises means for converting the actual noise level to an electrical signal, means for selecting electrical signals corresponding to noise levels above a given threshold level, means for analysing noise levels above the threshold level into a plurality of channels, means for producing a plurality of signals each corresponding to noise occurring in a particular respective channel and for weighting each signal according to the noise levels defining the respective channel and means for combining the weighted signals to form a combined signal representative of cumulative noise exposure.

The apparatus may include means for generating an audible or visual alarm signal when cumulative noise exposure over a given period exceeds e.g. a statutory maximum level. The apparatus may also include means enabling the display of the cumulative noise dosage and also for the display of the actual noise level at any given time.

Conveniently noise is detected by a microphone and noise levels above a desired minimum threshold cause the output of the microphone if desired suitably amplified, to be applied to a set of comparator circuits each corresponding to a particular noise exposure channel. The output of each comparator circuit may then be fed to an AND gate where it may be combined with clock signals from a standard clock circuit to give an output which is subsequently fed to a suitable digital/analog converter which produces an output voltage corresponding to the amount of noise exposure in the respective channel.The individual exposures are then summed in a suitable summing circuit to provide a voltage representative of the total exposure. Afurther comparator circuit may compare that total exposure voltage with a pre-set voltage and initiate an audible or visual alarm should a predetermined statutory maximum be exceeded.

The invention is illustrated by way of example with reference to the accompanying drawings in which: Figure I is a block diagram of an automatic multi-channel noise dosimeter according to the present invention, Figure 2 is a detail of the circuit of Figure 1 and Figure 3 is a further detail of the circuit of Figure 1.

Referring to the drawings, the apparatus is designed to analyse noise values in the seven channels identified above. In order to provide an input a microphone 1 is placed at the position where it is desired to monitor noise e.g. at an operators station for operating noisy machinery. The output of the microphone 1 is fed to a pre-amplifier 2 the output of which is fed to an automatic switch 3 via a circuit for generating a voltage corresponding to the root mean square value of the pre-amplifier output signal. This automatic switch circuit is shown in more detail in Figure 2 and it can be seen without difficulty that this consists effectively of three level detector circuits, each enclosed in a chain line box, each constituting a window detector.The outputs of these are fed via an attenuator and two differential amplifiers to a set of switches and depending on the noise level the signal is then fed to one of three frequency weighting filters A, B and C.

These filters are designed to internationally accepted standards and the construction of the filter circuit is shown in Figure 3. If the noise level is 55 dB or less, the signal passes through filter A, or for noise levels of 55 to 85 dB it passes through filter B. If the noise level is greater than 85 dB, it passes through filter C. The outputs of all three filters are combined in a summer 10 whereafter the true r.m.s. value of the output of the summer is formed in a further circuit. The output of this further circuit may be fed to a standard circuit arranged to produce a signal which, when displayed, gives the actual noise level as measured by the microphone. The display 14 may be a standard digital display provided with switching means enabling the actual noise level to be displayed or alternatively the cumulative noise dosage to be displayed.

In order to calculate the cumulative noise dosage, the output of the r.m.s. circuit is fed to seven comparators to each of which is fed a comparison voltage in accordance with the following Table which shows the noise level, the r.m.s. value of the microphone output as measured at point 12 and the comparison voltage applied to the comparator in question.

Noise level dB Rums. voltage at Comparator channel Comparison voltage point (volts) number (volts) 55 0.0113 85 0.356 1 0.36 90 0.632 2 0.63 95 1.125 3 1.13 100 2.000 4 2.0 105 3.557 5 3.56 110 6.325 6 6.33 115 11.247 7 11.25 117 14.159 The comparators each produce an output when the voltage applied exceeds the comparison voltage applied to the respective comparator and this output is fed to a respective AND gate, AND 1 to AND 7, to which clock pulses are fed at the same time. These clock pulses are generated from a real time clock which, if desired, may have an appropriate display and which produces a one cycle per second output which is then divided by 60 and applied to AND gates AND 3 to AND 7 inclusive.The clock pulses are further divided by 4 and 2 and pulse trains corresponding to one pulse every four minutes and one pulse every two minutes are applied to AND gates AND 1 and AND 2 respectively.

The output of each AND gate is fed to a digital/analog converter, forming one of a series of seven such converters DIA1 to D/A7 inclusive. These converters produce an output of voltage corresponding to the number of pulses which they have received from their respective AND gate and in accordance with the following Table which shows the channels, the noise level range covered by the channel, the allowable maximum exposure, the allowable equivalent maximum number of pulses at the particular pulse rate in question, the output voltage of the output of the D/A converter associated with the respective at the maximum exposure time for the respective channel and the full scale output in volts of the respective D/A converter.

Allowable equivalent DIA Converter Allowable max. no. of outputatmax. DIA Converter dB max. time pulses and Time T (hrs.J full scale Channel allowed T. (hrs.) rate (volts) output (volts) C1 # 85 16 240 at 1 1.0 1.0 x 255 pulse per 240 4 mins.

C2 # 90 8 240at1 0.5 0.5 x 255 pulse per 240 2 mins.

C3 # 95 4 240at1 0.5 0.5 x 255 pulse per 240 min.

C4 #100 2 120 at 1 0.5 0.5x255 pulse per 120 min.

C5 3105 1 60 at 1 0.5 0.5 x 255 pulse per 60 min.

C6 #110 2 30 at 1 0.5 0.5 x 255 pulse per 30 min.

C7 > 115 4 15at1 0.5 0.5x255 pulse per 15 min.

The outputs of the digital to analog converters D/A 1 to D/A 7 are all fed to a summer 18 which produces a summation voltage in accordance with the following equation: SN = C1 - C2 + C2 - C3 + ....+C6-C7+C7 T1 T2 T6 T7 In this equation C1 is the exposure period at the specified noise level and T1 is the maxmum exposure time at that level.

By using 8-bit D/A converters, and clock pulses at the rates indicated, the maximum number of pulses may be kept below 256 and the maximum voltage may be set at 1 volt. The voltage from the summer 18 can be displayed by means of display 14 and may also be fed to a comparator circuit 16 which is additionally in receipt of a comparison voltage of 1.0 volts. If the voltage corresponding to the noise dosage exceeds the reference voltage 1.0 volts, then an audible alarm may be initiated.

In a simplified version of the apparatus, the automatic switch may be modified so that only noise levels above 85 dB are passed for further processing. The automatic switch circuitry shown in Figure 3 operates with the conditions set out in the following Table: Output range of R.M.S. Active Attenuator Differential Differential Active Circuit 1 Equivalent window 1 output amp. 1 output amp. 2 output Active weighting (Volts) dB range detector (Volts) (Volts) (Volts) Switch network Vo < .006 dB < 50 1 +5 -0.7 0 1 A *.006#Vo < .0113 50#dB < 55 1 and 2 +5 0 -0.7 1 A .0113#Vo < .2 55#dB < 80 2 0 +5 -0.7 2 B *.2#Vo < .356 80#dB < 85 2 and 3 0 +5 +5 2 B .356#Vo#14.159 85#dB < 117 3 0 0 0 3 C * Overlap region

Claims (5)

1. Apparatus for detecting when noise exposure at a location, over a given period of time, exceeds a predetermined level, which apparatus comprises means for converting the actual noise level to an electrical signal, means for selecting electrical signals corresponding to noise levels above a given threshold level means for analysing noise levels above the threshold level into a plurality of channels, means for producing a plurality of signals each corresponding to noise occurring in a particular respective channel and for weighting each signal according to the noise levels defining the respective channel and means for combining the weighted signals to form a combined signal representative of cumulative noise exposure.

2. Apparatus according to claim 1 and including means for generating an audible or visual alarm signal when cumulative noise exposure over a given period exceeds a predetermined level.

3. Apparatus according to claim 1 or 2 and including means enabling the display both of the cumulative noise dosage and the display of the actual noise level at any given time.

4. Apparatus according to any one of claims 1 to 3 wherein the means for converting the actual noise level to an electrical signal comprises a microphone.

5. Apparatus according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.